/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 *
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package org.apache.skywalking.oap.server.configuration.zookeeper;

import org.apache.curator.RetryPolicy;
import org.apache.curator.framework.CuratorFramework;
import org.apache.curator.framework.CuratorFrameworkFactory;
import org.apache.curator.framework.recipes.cache.ChildData;
import org.apache.curator.framework.recipes.cache.PathChildrenCache;
import org.apache.curator.retry.ExponentialBackoffRetry;
import org.apache.skywalking.oap.server.configuration.api.ConfigTable;
import org.apache.skywalking.oap.server.configuration.api.ConfigWatcherRegister;

import java.util.Set;

/**
 * @author zhaoyuguang
 */
public class ZookeeperConfigWatcherRegister extends ConfigWatcherRegister {
    private final PathChildrenCache childrenCache;
    private final String prefix;

    // 构造方法中，初始化 CuratorFramework 这个 ZooKeeper 客户端连接指定的 ZooKeeper 集群
    public ZookeeperConfigWatcherRegister(ZookeeperServerSettings settings) throws Exception {
        // 监听周期
        super(settings.getPeriod());
        // ZK节点的前缀
        prefix = settings.getNameSpace() + "/";
        // 根据配置创建重试策略
        RetryPolicy retryPolicy = new ExponentialBackoffRetry(settings.getBaseSleepTimeMs(), settings.getMaxRetries());
        // 根据配置指定的地址，创建CuratorFramework客户端
        CuratorFramework client = CuratorFrameworkFactory.newClient(settings.getHostPort(), retryPolicy);
        // 开启与 ZooKeeper 集群的会话
        client.start();
        // 创建PathChildrenCache
        // PathChildrenCache 是 curator-recipes 中提供的一个缓存工具类，它会将指定路径下的全部子节点
        // 信息缓存到本地，并且监听这些子节点的变化。当监听到子节点的变更事件时，会将变更结果更新到本地缓
        // 之中。另外，可以向 PathChildrenCache 中添加 Listener 监听数据变化。
        this.childrenCache = new PathChildrenCache(client, settings.getNameSpace(), true);
        this.childrenCache.start();
    }

    // 直接从这个 PathChildrenCache 缓存中读取最新数据并整理成 ConfigTable 对象返回
    // 即可，60s 的定时轮训线程会执行 configSync() 方法之中完成新旧配置值的比较并通知
    // 相应的 ConfigWatcherRegister 对象。
    @Override
    public ConfigTable readConfig(Set<String> keys) {
        ConfigTable table = new ConfigTable();
        keys.forEach(s -> {
            ChildData data = this.childrenCache.getCurrentData(this.prefix + s);
            table.add(new ConfigTable.ConfigItem(s, data == null ? null : new String(data.getData())));
        });
        return table;
    }
}
